1. Field of the Invention
The present invention relates to a method of determining a merging bit, and more particularly, to a method of determining a merging bit which can make a digital-sum-value close to zero to comply with a run-length rule.
2. Description of the Prior Art
In the prior art rewritable optical recording system, 8-bit symbol data should be encoded in non-return-to-zero (NRZ) format and be transformed into 14-bit channel bit data, the two steps together being called eight-to-fourteen modulation. According to the Red Book, when the data “0” appears in a data stream of 14 bits, the run-length of “0” is limited to shorter than 11T (11 periods of a reference clock) and longer than 3T, this being called the run-length rule. The run-length is limited to maintain the linear speed of the compact disc according to the data stream of 14 bits. 3T represents a 720 KHz signal with the linear speed 1.2 m/s and 11T represents a 196 KHz signal with the linear speed 1.2 m/s, with any signal out of the standard timing being regarded as an error message.
In addition, the digital-sum-value is generated by the NRZ of the 14 bits. This helps the average potential of the NRZ of the 14 bits to be close to the DC potential. According to the Red Book, three merging bits must be inserted into any two data streams of 14 bits for complying with the run-length rule and for keeping the average potential of the NRZ of the 14 bits near the DC potential. The ideal merging bits should be carefully calculated.
The prior art requires four memories or registers to record the former 14 bits, the latter 14 bits, the digital-sum-value of the 14 bits and the NRZ level. A merging bit is determined by the data of the four memories according to a predetermined table. However, this requires more memory to store the predetermined table and a decoding circuit to calculate data, and thus, reduces the speed of the data process in the rewritable optical recording system.